How to make a foggy background
Another option is to surround your scene with a cube and give it a Volume based on a Volume Scatter node, with it's density controlled by the Z-Depth or View Distance output of a Camera Data node. In the example below, I used &quot;multiply&quot;, as this gives the best generalized effect, whereas, &quot;add&quot; will make the fog &quot;fill in&quot; the caverns as the camera scrolls out. Have a try with both if you like.

This is the scene in solid mode:

As you can see, all I did was surround the scene with a cube, remove the surface color from the cube, and connect the Volume Scatter with a low density (governed by the Math nodes)